A human body has a body temperature around 36° C. and the human body emits infrared light having a wide range spectrum of 2 to 30 μm in radiation emitted from skin. By detecting this light, it is possible to detect a position or motion of a human body.
Sensors operating in the above wavelength band of 2 to 30 μm include a pyroelectric sensor and a thermopile. In order to realize a high sensitivity in these sensors, it is necessary to provide a hollow region between a light receiving part and a light incident window, and thereby the sensors are prevented from being made smaller.
For solving the limitation by the hollow structure of the thermopile or the pyroelectric sensor, a quantum-type (photovoltaic-type) infrared sensor is expected. The quantum-type infrared sensor includes a PN junction photodiode structure formed with a junction of an n-type semiconductor in which majority carriers are electrons and a p-type semiconductor in which the majority carriers are holes, or a PIN junction photodiode structure having an intrinsic semiconductor between the p-type semiconductor and the n-type semiconductor. In the quantum-type infrared sensor, an electron-hole pair generated by a photon of the infrared light in a depletion layer existing in the PN junction or the PIN junction is separated spatially along a gradient of a valence band and a conduction band to be accumulated, and as a result the p-type semiconductor is charged positively and the n-type semiconductor is charged negatively to generate an electromotive force in between. This electromotive force is called an open-circuit voltage and can be read out as a voltage by the use of an external resistance (optionally, high input impedance circuit or amplifier) having a resistance value larger than the resistance of the PN junction or the PIN junction or can be read out as a current when short-circuited outside the quantum-type infrared sensor.
A problem when such a quantum-type infrared sensor is used as a human sensor at a room temperature is as follows. That is, a difference between an environmental temperature of human activity and a human body temperature is small and therefore output signal is small, and fluctuated infrared light radiated from the environment is detected by the sensor as noise and thus it is difficult to secure a sufficient S/N ratio. Accordingly, in a typical quantum-type infrared sensor, a light receiving part is cooled from an external temperature to increase the output signal and to increase the S/N ratio. As a representative of this quantum-type infrared sensor, a sensor using InSb as a semiconductor layered part, MCT (Mercury Cadmium Teluride), or the like is used.
In a quantum-type infrared sensor using the above compound semiconductor, as shown in Patent Literature 1, there is proposed a method of disposing semiconductor sensors in a planar state and taking out the output voltages of the sensors in a multi-stage series connection to improve the S/N ratio as a human sensor while realizing a smaller size without cooling.
As an application example of the above light sensor, there is expected to be realized a light receiving device for performing calculation of a light intensity radiated from a target object, or calculation of motion of a target object, or a distance to the sensor. In order to realize such a light receiving device, there is devised a method of using a difference value or a summation value of the outputs from a plurality of light sensors.
By using the difference value of the outputs from a plurality of the outputs, it is possible to detect the light receiving intensity of the light radiated from the target object and the motion of the target object. By using the summation value, it is also possible to detect approaching of human body. Further, by arranging the light sensors in an array and calculating a signal based on light receiving intensities of the light entering the sensors in a subsequent stage calculation circuit, it is possible to obtain a sum or a difference of the output signals from the sensors (refer to Patent Literature 2).
FIG. 13 shows a configuration of a conventional light receiving device calculating the difference value and the sum of the outputs from the outputs of the light sensors. FIG. 13 shows a conventional light receiving device 1300 including a light sensor part 1310 including light receiving parts dA to dD, a current-voltage conversion unit 1320 connected to the light sensor 1310 and including current/voltage (current-voltage) conversion amplifiers 4a to 4d, a differential operation unit 1330 connected to the current-voltage conversion unit 1320 and including a first subtraction circuit 5 and a second subtraction circuit 6, and a summation operation unit 1340 connected to the current-voltage conversion unit 1320 and including addition circuits 9 summing output signals from the current-voltage conversion amplifiers 4a to 4d. 
In the light sensor part 1310, one of the terminals of the light receiving parts dA to dD are connected to one of the input terminals of the current-voltage conversion amplifiers 4a to 4d via output terminals 3a1 to 3d1, respectively. The other terminals of the light receiving parts dA to dD are grounded via respective output terminals 3a2 to 3d2. In the current-voltage conversion unit 1320, the other input terminals of the current-voltage amplifiers 4a to 4d are grounded. An output terminal of the current-voltage conversion amplifier 4a is connected to the first subtraction unit 5 and the summation operation unit 1340, an output terminal of the current-voltage conversion amplifier 4b is connected to the second subtraction unit 6 and the summation operation unit 1340, an output terminal of the current-voltage conversion amplifier 4c is connected to the first subtraction unit 5 and the summation operation unit 1340, and an output terminal of the current-voltage conversion amplifier 4d is connected to the second subtraction unit 6 and the summation operation unit 1340. The summation operation unit 1340 sums four signals (outputs of the current-voltage conversion amplifiers 4a to 4d) with the addition circuit 9.
In the light sensor part 1310, the light receiving parts dA to dD receive light radiated from an target object, and output currents according to the intensity of the incident light to the respective current-voltage conversion amplifiers 4a to 4d via the respective output terminals 3a1 to 3d1. In the current-voltage conversion unit 1320, the current-voltage conversion amplifiers 4a to 4d convert the respective input currents into voltages and output the respective voltages to the differential operation unit 1330 and the summation operation unit 1340. In the differential operation unit 1330, the first subtraction circuit 5 calculates a difference between the outputs from the current-voltage conversion amplifiers 4a and 4c and outputs a subtraction signal, and the second subtraction circuit 6 calculates a difference between the outputs from the current-voltage conversion amplifiers 4b and 4d and outputs a subtraction signal. The summation operation unit 1340 outputs a summation operation result of the four signals (outputs of the current-voltage conversion amplifiers 4a to 4d).